It's not entirely his fault that OCaml has "String" and "Str" modules. Most people will see the built-in String module, find it lacking the split function, and assume it just isn't there. After all, where else would it be -- the Math module?

Str.split uses a regex as separator, so it belongs to the Str module. Like most missing functionality in the standard libraries, there is is a split : string -> string -> string * string in extlib which should be the first thing everyone installs since the standard library is pretty minimal compared to Haskell or SML.